Linux bridge transmit/ receive path

* I draw the call sequence diagram one year ago based on linux-2.4.27, but it should be alike in 2.6.

* __dev_queue_push_xmit in 2.4.27 ==> br_dev_queue_push_xmit in 2.6.16
* __br_forward_finish in 2.4.27 ==> br_forward_finish in 2.6.16
* there is no __br_dev_xmit in 2.6.16
*br_pass_frame_up_finish calls netif_receive_skb instead of netif_rx